Codd's Twelve Rules - Rule 3 - Systematic treatment of NULL values
Rule 3 | Systematic treatment of NULL values |
Rule | NULL values (distinct from empty character string or a string of blank characters and distinct from zero or any other number) are supported in the fully relational RDBMS for representing missing information in a systematic way, independent of data type. |
Description | This rule demands that we must be able to use a NULL placeholder irrespective of data type used. NULLs are distinct from an empty character string or any other number, and they are always to be considered as unknown values. This rule insists provisions for manipulating NULL values in all the possible ways. |
Example | SELECT Emp_Salary FROM Employee WHERE EPhone IS NULL; This query should return Emp_Salary if EPhone value is NULL/Not known. |
Some DBMS that fulfills this property | Oracle, SQL Server, and MySQL supports NULL values to be handled. They provide mechanisms to handle NULL values like other values. |
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